fix(reviewer-bot): harden stage2 readiness closure gates

[PLeVasseur]

Summary

• make the Stage 2 closure artifacts and dual-artifact gate machine-checkable with frozen green and blocked contract fixtures plus stale-attempt checks
• simulate the frozen deferred-payload residue classes and bind reconcile artifact selection to the retained zero-or-one JSON payload contract
• prevent late workflow_run reconcile from recreating removed review entries or sidecars after closed-path cleanup

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